Abstract Cystic Fibrosis (CF) is a genetic disorder affecting thousands in the United States and worldwide. Loss of function mutations in the CF transmembrane conductance regulator (CFTR), a chloride ion channel, results in abnormal maintenance of airway surface liquid and loss of mucociliary clearance. CF patients suffer from chronic bacterial infection in the lung with limited therapeutic recourse. Staphylococcus aureus is an important Gram-positive organism in the context of CF. With the increasing prevalence of methicillin-resistant S. aureus (MRSA), this bacterium will likely play a greater role in CF pathogenesis in the near future. MRSA colonization is associated with greater declines in lung function in CF and may influenza polymicrobial infections often found in this disease. Our laboratory has previously demonstrated an important role for Type 17 immunity in regulating S. aureus host defense. Type 17 cytokines, IL-17 and IL-22, stimulate antimicrobial peptide production by epithelial cells that are critical to S. aureus clearance. At this time, it is unclear if Type 17 or related antimicrobial responses are aberrant in the context of CF. Further, preceding viral infection increases susceptibility to S. aureus infection by suppressing anti-bacterial Type 17 immunity. Viral exacerbation in CF is associated with acquisition of bacterial infection. For these reasons we propose that CFTR deficient epithelial cells display attenuated responses to Type 17 cytokines resulting in worsened S. aureus infection and that influenza co-infection exacerbates S. aureus carriage. This hypothesis will be tested in two specific aims. First, we will investigate S. aureus biofilm growth on human airway epithelial cells from CF and control patients and the impact of Type 17 cytokines on colonization. Second, we will characterize the Type 17 immune response during nasal S. aureus infection in a murine nasal epithelial model of CF and examine how influenza affects S. aureus carriage. The proposed studies will examine the Type 17 immune response during S. aureus infection and how influenza co-infection affects S. aureus growth in CF. These studies will help clarify host-pathogen interactions in CF and characterize the relationship between two important respiratory pathogens that contribute to a decline in lung function in CF patients.